This study analyses how institutional factors impact on processes of isomorphism in the design of regulatory regimes. It does so through a comparative examination of regulatory reform in the railway domain in Britain and Germany in three time periods, the post-First World War and the post-Second World War periods as well as the 1990s. It is argued that pressures for isomorphism, defined as the increasing homogenisation of a unit with other units in its policy environment, are exerted by several policy environments. These pressures can be distinguished in their degree of domain- and paradigm-orientation. Domain-orientation consists of regulatory change which is based on sector-specific sources, whereas paradigm-orientation involves the application of supposedly universal 'policy recipes' across policy domains. The study questions whether three institutional factors - the insulation of the regulatory space from coercive pressures, the insulation of the political-administrative nexus in the regulatory space and the insulation of the regulatory space from societal forces - can explain why in some cases reforms are domain-oriented, but, in other cases, reforms are paradigm-oriented. The comparative analysis of reform in British and German railway regulation provides three conclusions. First, in all cases, pressures for isomorphism emerging from different policy environments provided competing 'templates' for regulatory design ideas. Second, among the institutional factors, the insulation of the political-administrative nexus in the regulatory space was identified as the most important factor for explaining the orientation of the selected regulatory instruments. Third, in the light of the study's historical and institutional perspective, this thesis critically evaluates arguments proclaiming the emergence of a 'regulatory state' in contemporary Europe.